Mechanical couplings for joining pipe elements together end-to-end comprise interconnectable segments that are positionable circumferentially surrounding the end portions of co-axially aligned pipe elements. The term “pipe element” is used herein to describe any pipe-like item or component having a pipe like form. Pipe elements include pipe stock, pipe fittings such as elbows, caps and tees as well as fluid control components such as valves, reducers, strainers, restrictors, pressure regulators and the like.
Pipe elements are vulnerable to failure at joints where two or more pipe elements are connected. Failure at a joint may be caused by applied forces inducing stresses in pipe elements or couplings that exceed the yield strength of the material forming the pipe elements or coupling, at much lower loads applied cyclically which induces fatigue failure, or when bending forces add additional stresses. Regardless of the failure mode, stress concentrations in the pipe elements can play a major role in limiting the performance of pipe joints, especially mechanical pipe joints assembled from polymeric pipes, as some polymeric materials are sensitive to stress concentrations.
Stress concentrations may be a factor, for example, when polymeric pipe elements are joined by mechanical couplings, commonly made of metal, having arcuate projections, known as “keys”, which engage circumferential grooves in the pipe elements. The grooves, whether formed by machining operations or cold worked (roll-grooved), will have regions of stress concentration in the “corners” of the groove, where the sides of the groove meet the floor of the groove, such regions being near or adjacent to the location of the engagement of the key within the groove, and thus, where forces and stresses are transferred across the joint by the coupling. Further, known engineering considerations governing the groove geometry and the groove/key contact tend to increase stress in the groove corners, these considerations including the known need to maximize the height of the engagement between the sides of the keys and the sides of the groove for performance purposes while minimizing the depth of the groove, and, for polymeric pipe, minimizing or eliminating compressive forces applied by the key to the bottom of the groove, which act to increase stress at the groove corners. Where other forces, such as bending forces, are applied to the joint and carried only by the key/groove interface, stress is further increased at the groove corners. At those corners, stress concentrations are formed as a natural consequence of the sharp corner or small radius present where the two surfaces meet. Such stress concentrations are exacerbated by the need, in practical pipe couplings, to account for manufacturing tolerances on the coupling and the pipe elements themselves. These manufacturing tolerances create geometric variability in each component, the accommodation of which limits the degree to which coupling designers can reduce the effect of stress concentrations. The increased stress in the groove corners and the stress concentrations naturally occurring there promote the formation of small cracks in some polymer pipe elements, which can lead to an ultimate failure under high applied loads, or fatigue failure under stress inducing cyclic loads. There is clearly an advantage to reducing or eliminating the increased stresses in the corners of the grooves in polymeric pipes and stress concentrations at those locations, including by reducing the effects of geometric variations on the key/groove interface.